General Considerations and Initial Procedures
Examine from a distance first
Pupillary reflexes to light
Evaluation for bacteria, fungi and viruses
Culture and sensitivity
Measurement of tear formation
Schirmer tear test
Localization of intraocular opacities
Direct versus indirect ophthalmoscopy
Principles of ophthalmoscopy
Interpretation of ophthalmoscopic appearance
Evaluation of vision
Evaluation of motion detection
Objective signs of sudden total blindness
Objective signs of gradual or congenital blindness
Pupillary responses and vision
Schirmer tear test
Nasolacrimal drainage system passage time
Lavage of nasolacrimal system
Examine for corneal ulcers
External ophthalmic stain for conjunctiva
Ultraviolet light for exam of lens
Recording of the electroretinogram
Clinical uses of electroretinography
Evaluation of the electroretinogram
Photography of the eye
Auriculopalpebral nerve block in horses
Pre-purchase exam in horses
The eye is unique in that most of its structures can be visualized and clinicopathologic diagnoses frequently are possible. In some cases, a lesion seen in a survey may suggest a specific systemic disease.
You must have a basic knowledge of gross and histologic anatomy of the eye and orbit in order to make a diagnosis and provide a prognosis. The figure linked in the previous sentence is meant to give you a sense of the relationship of the various ocular tissues to one another. You should consult an anatomy text for more detailed information. The circular inset in the figure is a magnified view of the posterior layers, with retina, tapetum, choroid proper and sclera, from left to right.
The adnexa and ocular anterior segment can be thoroughly examined in a lighted room with the aid of an examining light and source of magnification, such as the Opti-Visor loupe with dial adjustable headband . The ocular posterior segment can best be evaluated by ophthalmoscopy in a darkened room. Always examine both eyes even if only one seems abnormal. If the eye is painful to the patient and there is danger of injuring the eye during examination, or if the eye cannot be thoroughly examined, use chemical restraint or facial nerve blocks.
This section is intended to provide highlights to consider when examining the eye and is not intended to be complete. Not all ocular examinations require artificial mydriasis. Although ophthalmologists may routinely dilate pupils for examination, if you are dealing with conditions easily evaluated without mydriasis, then you may choose not to dilate. If you are evaluating the lens or ocular fundus, however, mydriasis is essential. In these situations, in order to save time, the pupillary responses to light could be checked right away and a dilating agent placed in each eye so that the pupils will dilate while you take a more comprehensive history and do the external exam. Evaluation of tear production or the taking of samples for bacterial cultures, however, may be more convenient to do before applying a mydriatic, if either of these is indicated. The iris should be examined before the pupil dilates completely.
In some cases, e.g., when glaucoma is suspected, pharmacologic mydriasis is contraindicated.
As with any other problem, a history is important to properly interpret your physical exam findings. This should include establishing how long the client has had the patient.
Establish when and under what circumstances vision became impaired or ocular problems began. The appearance of the eye, as perceived by the client, may give valuable clues as to what came first when dealing with an entity such as cataract. As an example, suppose a patient has dense, bilateral cataracts that prevent exam of the ocular fundi. If the client indicates that vision was not impaired until the pupils' became white, the cataracts may be the sole reason for vision loss. If, however, the client states that the pupils were normal when vision was first noticed defective, the cataracts may be hiding retinal degeneration.
Other questions might include:
These are only ideas. You should tailor your questions to fit the situation at hand.
Examine from a distance first
Pupillary reflexes to light
Normally bilateral regardless of eye stimulated depending on species. For example, some species of birds do not have a true (neuronal) consensual response; in some, because the orbits are separated by a thin plate of bone, a strong light in one eye may reach the opposite eye as well, stimulating that retina and simulating a consensual response; this is especially easy to do in birds having markedly divergent eyes; the amount of ocular fundus melanosis also may play a role. Rabbits and rodents also do not appear to have much of a consensual response.
The precise receptors for the pupillary response have not been discovered , but are presumed to be the rods and cones from which electrical impulses pass to bipolar cells. From bipolar cells, impulses continue to retinal ganglion cells whose axons constitute the optic disk and nerve. At the optic chiasm, part of the nerve fibers decussate; in most mammals, the fibers from the lateral part of the retina do not decussate much. The fibers continue from the optic chiasm as optic tracts (same axons). In the optic tracts, visual and pupillary fibers begin to differ in pathways. The visual fibers synapse in lateral geniculate bodies and go via optic radiations to the occipital cortex. The pupillary fibers continue to the midbrain to synapse in the pretectal nuclei (crossing occurs between the nuclei of the two sides). The pretectal nuclei fibers project caudally and synapse in the oculomotor nuclei (both oculomotor nuclei receive axons of neurons from each pretectal nucleus). Preganglionic parasympathetic axons go from the oculomotor nerve and synapse in both ciliary ganglia.
Postganglionic parasympathetic fibers innervate the iris sphincter and ciliary muscles (note: some parasympathetic fibers also innervate the iris dilator muscle, but probably have an inhibitory function).
Sympathetic innervation of the iris is necessary to provide control of pupil size by providing constant antagonism to the sphincter via the dilator muscle. The sympathetic impulses originate in the hypothalamus and rostral midbrain, pass to and synapse in the intermediate gray column of the first three thoracic segments of the spinal cord. Preganglionic sympathetic neurons pass to the thoracic sympathetic trunk, go cranially (usually without synapse in the intervening cervicothoracic and middle cervical ganglia) to enter the cervical sympathetic trunk. At the cranial cervical ganglion synapse occurs and postganglionic fibers concerned with pupil size pass to the dilator muscle of the iris (note: some sympathetic fibers have been found in the sphincter muscle and probably have an inhibitory function).
In ophthalmology, we often refer to afferent and efferent components of the pupillary responses. By afferent we mean the entire sensory pathway and by efferent we mean the entire motor pathway. Therefore, the afferent branch would include things like the retina and optic nerve up to the mid-brain region and the efferent branch would include the oculomotor nerve and the iris sphincter muscle.
When a particular eye is stimulated, the response seen in that eye is the direct response in that eye and is referred to as such (e.g., the direct response of the right eye). There continues to be some confusion, however, over how pupillary responses are defined with respect to the reaction seen (or expected) in the non-stimulated eye.
The confusion stems around which eye is referenced with respect to the response seen in the non-stimulated eye. Is it the indirect or consensual response of the stimulated eye or is it the indirect or consensual response of the non-stimulated eye? None of the standard textbooks I have consulted, human or veterinary, make an explicit statement in this regard. This issue becomes important in how we convey our results to others and how others interpret this.
If I say to you that shining a light in the right eye of a patient results in normal constriction of the right pupil, but that no constriction of the left pupil is seen, you would conclude that there likely is an efferent defect in the left eye. If I then tell you that shining a light in the left eye of this patient fails to cause a constriction of the left pupil, but does result in a normal constriction of the right pupil, you now are convinced (or should be) that the defect lies in the efferent arm of the left eye's pupillary response pathway (i.e., there is something wrong with the left oculomotor nerve or iris).
It is clear in the above example that the patient in question has a normal direct pupillary response in the right eye and lack of a direct response in the left eye. So far so good.
What is not clear is how I describe the indirect or consensual response. Do I say that this patient has no indirect response in/of the right eye (meaning that stimulation of the right eye fails to cause a response in the left eye)? Or, do I say that this patient has no indirect response in/of the left eye (meaning that the left eye fails to respond when the right eye is stimulated)?
Although I used to use the first definition, because it was the way it was taught to me, there is far from unanimity amongst ophthalmologists about which definition is correct. And, as I stated, the various textbooks do not make this issue explicitly clear. Although somewhat cumbersome, it is probably best if one simply states what happened if there is deviation from the normal or expected response. That is, in the case cited above, When the right eye was stimulated, the right pupil constricted, but the left did not. When the left eye was stimulated, the left pupil did not constrict, but the right did. This is unequivocal and clear.
From a clinical standpoint, it might even be better to go one step further and use your findings to make a conclusion. In the case cited above, you might write, Based on the pupillary responses to light, there was an efferent defect on the left side. The latter avoids definition ambiguities and provides clinically useful information.
In summary, make sure you understand the anatomic pathways for the pupillary responses to light. In this way you can localize lesions when you detect abnormalities in the pupillary response. Record the information about pupillary responses to light in your patients' records in a way that all can understand rather than relying on a simple, universal definition (which currently does not exist). Bear in mind that others may have to read and interpret your statements if patient care gets transferred to them.
- Miosis - constriction of the pupil.
- Mydriasis - dilatation of the pupil.
- Anisocoria - unequal pupil size - from aniso- (prefix meaning unequal) and -coria (suffix meaning pupil).
The direct pupillary response is done by simply shining a bright light into each eye. It may help to point the light towards the lateral aspect of the retina because this may be more sensitive, but this is unnecessary. It is best to do this under standard exam room lighting (i.e., not with the lights off) in order to also evaluate symmetry of the pupils without the complication that darkness would inject through reduction of parasympathetic tone.
It is often difficult to determine the response in the non-stimulated eye because exam room lighting reflects off the cornea preventing adequate visualization of the pupil. You can overcome this by one of the following methods:
- Use a direct ophthalmoscope after you have evaluated the direct response in each eye with the exam room lights on. Dim or extinguish the exam room lights and stand back far enough so that you can see both pupils via the fundus reflection when you look through the ophthalmoscope set at 0 diopters. Have an assistant shine a light first in one then the other eye while you observe the response in the non-illuminated eye.
- A more simple method does not require an assistant or extinguishing of exam room lights. First establish that each eye has a direct response. Then shine your light in the right eye. When the pupil constricts (or after a second or two, if the pupil does not constrict), quickly swing your light to the left eye. If there had been a constriction on the left side, the left pupil should still be in a constricted state if you have swung the light quickly enough to the left eye. (If you were not quite quick enough, the left pupil will be slightly dilated and will, if normal, constrict directly in response to your light.) Repeat the procedure for the left eye.
This is termed the swinging flashlight test. It is useful in identifying unilateral pupillomotor problems. For example, if you are going from the right pupil to the left pupil and the left pupil dilates rather than remaining constricted, this indicates a defect in the left eyes afferent branch of the pupillary light response pathway.
- With unilateral retinal damage, pupils usually normal size and equal because of intact pathways.
- With bilateral extensive retinal lesions, pupils usually dilated and incompletely responsive or unresponsive.
- With unilateral oculomotor nerve damage, affected pupil widely dilated and normal pupil usually more constricted than usual.
- In blindness due to occipital cortex lesions (or other lesions not affecting the pupillary pathway), the pupils and pupillary reflexes will be normal.
- If blindness and dilated pupils are present (with normal ocular fundi), lesion probably in optic nerves or optic chiasm.
Dilating the pupil:
Use 1% tropicamide in most species for routine mydriasis; it is a synthetic atropine derivative and acts by blocking the use of acetylcholine, therefore blocks parasympathetic action and pupil dilates due to sympathetic tone; pupil dilates in about 15 minutes in small animals, 20-25 minutes in large animals; lasts about 4-8 hours. Can be reversed with pilocarpine. If the patient has uveitis, tropicamide may not be adequate for dilatation.
Although atropine sulfate (one percent most common) can be used, its effects can last up to a week and are not easily reversible pharmacologically. Use only if tropicamide is inadequate.
Some rabbits have an enzyme in their aqueous humor which blocks atropine, therefore must use tropicamide or 10% phenylephrine.
Birds and reptiles have striated sphincter muscle and atropine will not work. Although some neuromuscular blocking agents may be used topically for mydriasis in birds, their use usually is unnecessary for routine examinations. More importantly, some of these agents can cause incapacitation of some birds, with death as a result. I no longer advocate the use of these agents because of this. It is usually possible to get an adequate view of the bird fundus if one's technique is good. If this is not possible, I believe that the risk to the bird may not be worth whatever information you may be able to get from mydriasis. For those of you who insist on using a dilating agent in birds, I recommend you read the articles on the subject in the Literature Cited section (Mikaelian, et al. and Ramer, et al.).
Anatomy and function:
Optic (II) - vision and pupillary response (afferent branch).
Oculomotor (III) - general movement of globe, retraction of globe, raising of upper eyelid, pupillary response (efferent branch).
Trochlear (IV) - extorsion of globe.
Trigeminal (V) - sensory supply to globe and peribulbar skin; lacrimal nerve is branch of ophthalmic nerve from trigeminal, and carries parasympathetic fibers to lacrimal gland; corneal reflex, palpebral reflex.
Abducent (VI) - lateral deviation of globe, retraction of globe.
Facial (VII) - close palpebral fissure; palpebral reflex.
Tonic eye reflexes:
Movement of the eye when the head is moved - used in assessing extraocular muscle function; may be helpful in localizing central nervous system lesions.
Some optic tract fibers continue to superior colliculus - fibers from superior colliculus go to cervical ventral motor cells and cranial nerve nuclei. Fibers from third, fourth, and sixth cranial nerve nuclei innervate the extraocular muscles as listed below:
- Oculomotor (third) nerve innervates: Dorsal rectus, Medial rectus, Ventral rectus, Ventral oblique, Levator palpebrae superioris, Retractor bulbi.
- Trochlear (fourth) nerve innervates: Dorsal oblique.
- Abducent (sixth) nerve innervates: Lateral rectus, Retractor bulbi.
Involves trigeminal (fifth; sensory) and facial (seventh; motor) nerves - any painful stimulus to or touching of cornea leads to reflex closure of palpebral fissure through action of orbicularis oculi muscle; have a direct corneal reflex (reaction of stimulated eye) and consensual corneal reflex (reaction of contralateral eye).
Similar to corneal reflex.
Also known as blink reflex; involves optic (second; afferent) and facial (seventh; motor) nerves - subcortical reflex from sudden stimulation of visual system (such as foreign body moving toward eye) - leads to reflex closure of palpebral fissure and turning of head away from the stimulus; may have a cortical component because it requires an intact visual and motor cortex on ipsilateral side. The clarity of the ocular media and the contrast of the menacing object with its surroundings are important considerations in interpreting this response. For example, if the patient has a complete cataract, the lack of a menace response is of little practical significance. Furthermore, vision and the menace response are not directly related. I have seen patients who had vision, but did not have a menace response and others have seen the reverse.
This is the involuntary avoidance response to a light shined into the eye. Especially when a strong light is directed at the eye, there will be blinking, retraction of the globe with protrusion of the third eyelid (in those individuals having the necessary anatomical features) and sometimes movement of the head away from the light. Although the neuroanatomical pathway for this response is not completely understood, a positive response generally is considered to be evidence of fundamental continuity of the eye to brain pathway and is suggestive that there may be the ability for vision. I believe that it is a considerably more reliable indicator of the potential for vision than the menace reflex and is particularly useful in situations in which the ocular media are opaque. The presence of complete cataracts, corneal scarring and so forth would not be expected to prevent this response.
There is no need to do cultures in most simple cases of conjunctivitis or keratitis (including ulcerative keratitis). If you suspect a pathogenic microorganism is present or if you continue to get a suppurative response despite antimicrobial medication, you may need to culture and determine which medication might be indicated.
Culture swabs or scrapings may be made from the palpebral (outer) conjunctiva of the third eyelid, margin of the eyelid, inner surface of the eyelid and fornix (conjunctiva) or directly from the cornea.
If the patient has been treated for a bacterial infection for more than a few days and the condition has not improved, there may be no need to wait for drug residues to disappear. However, growth of the organism may be inhibited on the plate in spite of its resistance in the eye; ideally, it would be best to discontinue treatment for several days before culturing.
Numerous species of bacteria normally live or are present within the conjunctival region. In order to determine if a bacterium is part of the normal flora of the patient being examined, you may want to culture both eyes for comparison. This would be done only in cases where you are unconvinced that there may be a bacterial infection, but are trying to rule out the probability of this.
Can use Kimura platinum conjunctival spatula or any small scalpel blade; with the scalpel blade, peel back the wrapper so that the handle end protrudes and then use the wrapper as a means of holding the blade end - this provides a sterile, disposable tool. Evert the upper eyelid and scrape to get material for a slide for microscopic exam - make at least two slides so you can do a Wright's and a Gram stain; inoculate a suitable culture medium with the remainder. Can use topical anesthetic to facilitate scraping (it is unlikely that the preservatives in the anesthetic will interfere with culture attempts).
Not routinely done, but can be important in making a quick diagnosis in some situations. Most useful in determining mycotic and mycoplasma infections or types of neoplasms.
This test is indicated when lower than normal tear production is suspected. If you or the client happen to put mydriatic drops or other liquid in the eye prior to doing this test, do not worry. Simply wait about ten minutes for the liquid to clear and do the test.
Best to buy commercial strips which can be obtained from most medical suppliers .
You could make your own from number 40 Whatman filter paper (should be 5 x 40 mm strips with a notch 5 mm from one end). If you do this, I suggest you try the strips on a few obviously normal dogs or cats to determine what normal' is for those strips.
No topical anesthetic should be used for the conventional test. In this case, the tear production being measured is a combination of what is present in the tear film and what is produced in response to any irritation caused by the diagnostic strip. This is known as the Schirmer I tear test in contrast to the Schirmer II tear test .
Place notched tip between lower eyelid and cornea . Leave in place one minute. You may have to gently keep the eyelids closed .
Measure length of moisture on strip excluding the tip that touched the eye. Measure immediately after removing from the eye or mark with a pen; the tear track can move several millimeters in a short time. At least one commercial source of these strips (by Schering-Plough) uses millimeter ruling and a dye which is activated by the moisture of tears so that measurement is facilitated .
Both eyes should be done for comparison. Always correlate your findings with clinical signs because some patients, particularly cats, may have lower than normal' values, but do not have ocular disease. In addition, Schirmer values fluctuate widely during the day and the figures quoted below represent a rough average. All values below are derived from one minute of contact with the strip.Cats and dogs:
- 9 mm or more is normal; most exceed 17 mm.
- 5-8 mm is probably decreased, but can have as little as 5 mm and appear healthy; cats in particular can have very low.
- Less than 5 mm indicates decreased formation; many patients with keratoconjunctivitis sicca will not moisten even the tip touching the eye.
- Normal range appears to be 0-11 mm (Abrams, et al.).
Cattle, goats, horses, pigs and sheep:
- Most have abundant tears (Marts, et al.).
- 15 mm or more is normal (usually much more).
- 10-15 mm is questionable.
- Less than 10 mm is abnormal.
As with section I, this section is meant to provide highlights.
This exam is facilitated with a penlight or portable operating light and a source of magnification (otoscope head such as Welch Allyn's catalog number 20200 with wide, swing out lens is good; slit-lamp or biomicroscope is best, but expensive - at least $1600). The patient should be handled so as to keep excitement to a minimum. Examine both eyes even if only one eye is involved. There is no special order to this, but you should develop a pattern that is followed so that important items are not missed.
Most of this phase is like any other physical exam. You are looking for deviations from normal. Examine each area of the eye, being careful to note inflammation, new growths, abnormal anatomy (developmental or positional), optical quality of those tissues requiring transparency, presence or absence of normal moisture, foreign bodies, and evidence of trauma, to name a few. You may want to measure and record the size of growths for future reference using something like a Jameson caliper .
Especially note the presence or absence of pain evidenced by blinking, increased sensitivity to touch, or self-trauma.
Pharmacologic pupillary dilatation is needed to adequately examine all intraocular structures deep to the iris.
Consider the posterior area of the lens nucleus as the center of rotation of the eye:
Purkinje-Sanson images - outdated method; may be state board question; generally thought of as three images: cornea and anterior lens surfaces act as convex mirrors (give erect, magnified virtual images) and posterior lens surface acts as concave mirror (gives inverted, minified real image). A fourth image due to reflection from corneal endothelial layer can be seen with the slit-lamp.
Best to have dim or no room light. Use mydriatic to dilate pupil. Ophthalmoscopy is needed for the posterior segment (deep vitreous to optic nerve). Two different types of ophthalmoscopy: direct and indirect.
Keep in mind that these two techniques are complementary. The fundus can be surveyed by indirect ophthalmoscopy to localize lesions, and the direct ophthalmoscope can be used to examine them more closely.
Advantages of direct ophthalmoscopy: Requires less practice; less cost; more magnification (varies inversely with size of eye; e.g., about 20 X in the cat and about 8 X in the horse); can be used with miotic pupils; useful in all species.
Advantages of indirect ophthalmoscopy: Survey instrument with less magnification (varies inversely with the dioptric power of the lens used and inversely with the size of the eye; e.g., about 4 X using a +20 diopter lens in the cat), but wider field allowing more fundus to be visible at one time (35 degrees instead of 9 degrees) - facilitates exam of peripheral part of fundus; permits exam at safe distance from fractious patients; provides stereoscopy; brighter illumination (for translucent ocular media); excellent teaching device because a prism can be used to give students a simultaneous view of what the examiner is seeing.
Direct ophthalmoscopy: Produces erect image.
Description of instrument :Most direct ophthalmoscopes have a large upper lens wheel containing plus and minus lenses of various dioptric powers with an illuminated window indicating which lens is in use (generally, red numbers indicate minus and black numbers plus).Technique:
A small wheel (either parallel with or perpendicular to lens wheel) contains various apertures through which the fundus is viewed.
- Slit - may be useful in determining whether a structure or lesion is elevated or depressed with respect to the plane of the retina.
- Small aperture - useful for neonatal or small eyes, or small pupils.
- Large aperture - generally used for routine ophthalmoscopy.
- Grid - useful in measuring relative size of a lesion and following its progress.
- Green filter - provides red-free light - nerve fiber layer more easily seen; supposedly helps to distinguish new hemorrhage from old blood (as in regions of hemorrhage).Set lens wheel at 0 diopters.Appearance of fundus:
When examining the patient's right eye, use your right hand to hold the ophthalmoscope, and use your right eye to look through the ophthalmoscope. Do the opposite for the patient's left eye. This makes it easier to get around the patient's nose. It is desirable to keep both eyes open when examining; once you learn to suppress the image coming from the eye not being used, keeping both eyes open makes the exam more easy by minimizing accommodation (your eye should be relaxed while looking through the ophthalmoscope).
Hold ophthalmoscope as close to your eye as possible (if wearing spectacles, hold against the spectacle lens).
From a distance of about 30 cm, find the fundus reflection through the pupil and then bring ophthalmoscope (and your eye) to within a few cm of patient's eye, all the time keeping your vision on the fundus reflection. At this time the structures of the ocular fundus should be visible; adjust lens wheel as necessary for focus (until you are experienced at this, however, lack of focus may be due more to improper technique than anything else).In general, retina and optic disk are at same level and focus (except in horses in whom the optic disk is slightly depressed). If retina is elevated, a more positive lens is necessary with the direct ophthalmoscope; lesions below the retina (or excavations) require a more negative lens.
Indirect ophthalmoscopy: Produces an inverted and reversed image
Description of instrument:Indirect ophthalmoscopes come as complete sets including headset with light source and viewing optics, transformer, and condensing lenses. These are ideal, but expensive (upwards of $1,000 in 1998). You need not have an expensive instrument to get an adequate survey of the fundus; you can use an inexpensive glass or plastic lens that has one flat and one convex surface (plano-convex lens), and which has a power of about plus 20 diopters; the lens should be 35 to 45 mm in diameter to be comfortable to hold; the light source can be a strong penlight, or the otoscope or ophthalmoscope portion of a diagnostic kit.Technique:
An inexpensive, but adequate lens is a 5X aspheric one made by Bausch & Lomb (Packette® Pocket Magnifier, Sight Savers® brand, Order No. 81-31-33). The lens is biconvex, 1.4" in diameter, about +20 diopters and comes in a self-contained plastic holder and cover. What it lacks in optical quality it makes up for in price (about $12.00 in 1998) and convenience. It matters not which side is facing the examiner. Various general merchandise or office supply stores may carry it or you can contact the manufacturer.
Better lenses which are made specifically for indirect ophthalmoscopy are ground to correct for spherical aberration and are optically coated to minimize reflection. They also are expensive ($90-$200). One of the least expensive of these good lenses is the Volk Conoid® produced by Tech Optics, Inc., USA, 7255 Industrial Boulevard, Mentor, OH 44060, price about $160 as of 1992.Stand at arm's length from the patient . Hold the lens in one hand with the convex side facing you; a finger of the same hand is placed on the patient's face to provide support and keep the lens a constant distance of several cm from the patient's eye ; the distance will be different for lenses of different sizes and dioptric powers, but the correct distance is that point at which the fundus image is all that you see in the lens (no eyelids, iris, cornea). The light source is held in the other hand so that the light path is directed at the pupil, and is parallel with and close to your viewing axis; support the light against the side of your head to stabilize it. If using an otoscope head, the magnifying lens can be swung out of the way and the examination done through the opening created.
Although it takes more practice, you should try to use the lens in your left hand for the patient's right eye and your right hand for the patient's left eye; this makes it easier to get around the nose.
Frequently, interpretations during ophthalmoscopy are based on the absence or displacement of structures, or their lack of visibility or lack of focus with surrounding structures. In most nonhuman patients, the eyes constantly move making focus on a particular area difficult - use ocular movements as a benefit - do not chase lesions. This is where indirect ophthalmoscopy is invaluable in surveying the fundus whereas with direct ophthalmoscopy, a small lesion might be missed due to eye movements.
Ophthalmoscopic exam involves: tapetal and nontapetal fundus areas in which lie the retina, retinal blood vessels, optic disk, tapetum, and choroid proper. Sclera often is visible when there is less melanin than usual in the retinal epithelium and choroid.
Nontapetal fundus often is erroneously called tapetum nigrum (this term is a misnomer because there are no tapetal cells present). The color of this region ranges from dark brown to red depending on species and breed - the color is due to retinal epithelial and choroidal melanin content (which is related to melanin distribution within the iris), and underlying blood vessels.
The best way to evaluate vision in a patient with whom an illuminating conversation is unlikely is to observe her or his behavior. Pupillary responses to light, the menace reflex, the dazzle' response and even the ability to respond to a moving object in the visual field do not measure vision. Instead, they evaluate the integrity of certain neuroanatomical pathways. All these can be present yet the patient still be unable to avoid obstacles or navigate.
A simple obstacle course should be used. However, some patients, especially cats, will not be cooperative.
The course should be run under daylight (photopic) and dark-adapted (scotopic) conditions to assess cone-mediated (photopic) and rod-mediated (scotopic) vision. A red light is useful for simulating scotopic conditions.
Cats are very difficult to evaluate for vision loss. A cat can be placed alone on an exam table and observed for cautiousness in jumping to the floor. If the cat does jump, watch closely the landing for signs of awkwardness or obvious unawareness of her or his environment.
If one eye only is suspected of being blind, the obstacle course should be run with one eye patched. Both eyes should be evaluated in this manner because some patients refuse to move with a patch regardless of their vision status.
Waving a hand before the eye may not be acceptable because movement of air causes most animals to blink regardless whether vision is present or they detect the motion by sight.
Use a clear plastic barrier to get reliable results.
A tuft of cotton dropped in front of the patient is helpful because little sound or air disturbance is created. Most sighted patients briefly will follow the cotton. Birds may be refractory to this.
Total blindness of sudden onset usually results in slow or cautious movement, or bumping into objects.
Many of these patients appear to have some sight because of increased perception by other senses (hearing and smell). He or she will memorize surroundings and move around well. Some horses will lead or respond so well to a bridle that they do not appear to be blind.
The lesson here is simple:
Smears represent exfoliated cells whereas scrapings are similar to superficial excision of tissue (Murphy). After topical anesthesia, cotton swabs are used to collect exudate and cells for smears. If there is extensive exudate, the conjunctival cul-de-sac should be flushed prior to scrapings which are taken using the Kimura platinum spatula or handle end of a scalpel blade.
The material is smeared onto glass slides and air dried, or fixed with acetone, alcohol or spray fixative.
Stain with Gram's, Wright's, Giemsa's, or modified Sano's techniques.
In bacterial infections, smears contain increased numbers of neutrophils and goblet cells. Greater numbers of eosinophils, basophils, and enlarged epithelial cells are seen in allergic conditions. Lymphocytes and plasma cells are more numerous in viral infections.
Use fluorescein strips to stain tear fluid and note appearance at external nares; may need to add a drop of artificial tears if the patient's eyes are not very moist; important to compare both sides; it takes up to two minutes in the dog or cat, and three to five minutes in the horse for the stain to appear at the external nares; not completely reliable in brachycephalic dogs or cats because may exit into pharynx; UV or cobalt blue light enhances visibility of the dye. If you are using UV light, shine it on the face before applying fluorescein to see if there is any fluorescent material already there, although this would not normally fluoresce like fluorescein.
This should be done before attempting to flush (lavage) the system.
Anatomy: Two puncta, two canaliculi, lacrimal sac, and nasolacrimal duct (rabbit only has lower punctum and canaliculus; pig only has upper punctum and canaliculus); puncta are located at mucocutaneous border of the upper and lower eyelids near the medial canthus .
Nasolacrimal flush: In horses retrograde lavage is done - head restraint and twitch usually sufficient; use topical anesthetic around orifice of nasolacrimal duct (floor of nose at mucocutaneous junction ) - cannulate with tomcat catheter, beveled polyethylene tubing, or blunted 18 gauge needle - flush with at least 10 ml water or saline; you may need to press down over the entrance as you flush if resistance forces fluid out the external meatus .
Lacrimonasal flush: In dogs, cats, and cattle; use human lacrimal cannulas or blunted needles (dog - 22-23 gauge, cat - 24-25 gauge, and cattle - 18-20 gauge; you can also make your own); use topical anesthesia; dogs and cats should be placed on their side and be firmly restrained (cats may need to be anesthetized or deeply sedated). Use tap water (you could warm it, but this may not make a noticeable difference), or artificial tears or sterile saline as the flushing medium. One hand should hold open the eye, one digit rolling the lower eyelid out so that you can visualize the lower lacrimal punctum. You should hold the syringe in such a manner that you can inject the fluid immediately entering the upper punctum; you should not have to reposition fingers at that time. Always rest the hand holding the syringe against the patient's head, or against your other hand which is against the patient's head; in this way, if the head moves, the syringe and cannula will move with it, and the chance of serious trauma will be minimized. Enter the upper punctum and canaliculus (except in the rabbit). Establish patency of the connection of the upper and lower canaliculi, and of the lower punctum, by injecting a small amount of fluid; the fluid should exit from the lower punctum (it also may exit from the nares). If it does not simultaneously exit from the nares, apply gentle pressure to the medial canthus through closed eyelids and inject more fluid until it does, or until you are convinced there is a blockage. If the patient swallows several times during this procedure, this is a sign of patency even if the fluid does not exit from the nose.
Fluorescein dye will stain regions where corneal epithelium is absent . Always use prepackaged sterile strips . Although fluorescein is available as a solution, solutions should never be used as they can lead to contamination and loss of the eye.
It is generally a good idea, if you are not an expert, to get in the habit of using fluorescein to test every eye which has signs of pain. Superficial ulcers may almost be invisible without fluorescein.
Simply touch the tip of the strip to the bulbar conjunctiva, but just fleetingly or else you will flood the eye with more dye than you need and make interpretation more difficult. There is almost never a need to otherwise moisten the strip, however if the eye is particularly dry, then a drop of artificial tear solution can be applied to the strip first. If too much is applied, flush excess stain with several drops of artificial tears prior to exam. Examine soon after instilling because the dye is water soluble and will rapidly diffuse into corneal stroma and anterior chamber if an ulcer is present. Ultraviolet light or cobalt blue light enhances visibility of the dye.
Do not touch the cornea with the strip because this may leave a spot of stain that can be confused with ulceration.
Be aware that fluorescein will interfere with the antibody test for herpesvirus infection.
Rose bengal dye - vital dye for staining dead or degenerating cells and mucus; excellent stain for demonstrating sick cells due to sicca (drying); stains lesions bright red; use prepackaged sterile strips.
The lens capsule will fluoresce faintly when ultraviolet light is used. This faint glow is distinct from other structures such as the cornea or iris. Ultraviolet light can, therefore, be helpful in determining the location of the lens, and in diagnosing ectopia lentis.
Measurement of intraocular pressure. Intraocular pressure normally varies with pulse rate, eyelid pressure, extraocular muscle tension, respiration, venous pressure, time of day, and blood osmotic changes. The pressure necessary to maintain the globe as a functional sphere results from the balance of the production of aqueous humor and the outflow resistance in the ciliary cleft (also known as filtration or drainage angle).
Eyes must be positioned so that the Schiøtz tonometer can be placed on the central region of the cornea while being oriented vertically . The Schiøtz tonometer is a required instrument in any small animal practice because it will likely be the only way in which you will be able to effectively rule in or out the diagnosis of glaucoma in patients who have conditions causing similar signs.
Principle and mechanics:
Cornea is indented with a weighted, relatively frictionless plunger having a constant area with a footplate whose curvature corresponds roughly to that of cornea; amount of plunger protruding from footplate depends on indentability of cornea - plunger is connected to scale whose values can be converted to mmHg (the higher the scale reading, the lower the intraocular pressure).
Use topical anesthesia; the 7.5 gram weight provides the most realistic readings; the patient should be placed on her or his back and kept calm; hold open the eyelids a distance from eyelid margins so as not to put pressure on the globe; be sure not to apply any pressure on the neck so that there is jugular resistance; the tonometer must be held vertically on the center of the cornea. Take two or three readings (all these should be close if your technique is correct). I believe the readings should be recorded as scale values over the weight used, rather than the actual pressure. Normal readings generally are in the range of about two scale values below or above the number represented by the weight used. Therefore, using a 7.5 gm weight, you would expect the normal range to be about 9.5 (low normal) to 5.5 (high normal) on the scale. The actual pressure range in normal cats and dogs is about 15 to 30 mmHg. Although there are conversion charts for dogs, they are not accurate and should be avoided. The human conversion chart which comes with the tonometer appear to be reliable for dogs and cats who have normal intraocular pressure (Miller and Pickett, a, b), however it is not known whether it would be reliable with respect to precise values in the case of elevated pressure. Practically, this likely is moot because the demonstration of elevated intraocular pressure, not the exact value alone, is what you should use to make the diagnosis.
Readings are invalidated in the following instances:
- Excessive struggling by the patient or excessive pressure on eyelids (therefore indirectly on the globe), artificially increasing the intraocular pressure.
- Curvature of cornea must be constant - not accurate when there is megalocornea or buphthalmia.
- Non-vertical positioning of tonometer.
- Severe corneal edema.
In principle, measures the amount of pressure needed to flatten a specific area of cornea. There are various types which are useful in veterinary ophthalmology (Miller, et al., a, b). Portable, hand-held models are particularly useful, but very expensive. Those used with a slit-lamp have little or no value in veterinary ophthalmology.
Digital measurement of intraocular pressure through upper eyelids; is unreliable and crude at best (can only detect large differences between two globes).
Place two fingers (not thumbs) on closed upper eyelid and apply sufficient pressure to indent globe - always compare both eyes. Now, forget what you just read and do not use this method (seriously).
Limited to experimental use - directly measures intraocular pressure in anterior chamber.
Gonioscopy is the exam of the iridocorneal angle (ciliary cleft) . This is particularly important in many patients who have glaucoma or who are suspected to have glaucoma.
Special lens called goniolens is used to facilitate viewing of the angle . Various types of goniolenses are available - e.g., Koeppe or Franklin.
After the application of a topical anesthetic, the lens is placed on cornea, and space between lens and cornea is filled with artificial tears or special gel. The angle is examined using a good light source with magnification (e.g., otoscope, or loupe and penlight).
Cats and horses have deep anterior chambers and their angles sometimes can be examined without a goniolens by using an otoscope head to look across the eye from the limbus. The gonioscopic lenses used for dogs will give a better view and should be used for more critical evaluation.
The retina, like other nervous tissue, generates electrical currents. A large potential exists between the fundus and the cornea with cornea being more positive. When a flash of light strikes the retina, rapid changes occur in retinal potentials which are recorded as an electroretinogram (Aguirre).
The electroretinogram differs between species depending on whether retinal photoreceptors are predominantly rods or cones. Polygraphs (usually electroencephalographs), or oscilloscopes are used to record the potentials produced by flashes of light from photostimulators. Tests are conducted under photopic and scotopic conditions.
Exact sites for each electroretinogram component (a, b, and c-waves) have not been determined with absolute certainty. However, the a-wave probably originates from photoreceptors, the b-wave from bipolar cell layer, and the c-wave from retinal epithelial cell layer. The electroretinogram is an algebraic sum of several components arising from different retinal layers.
Veterinary patients generally must be completely anesthetized for this test. An electrode is placed in contact with the patient's cornea, and a second electrode is placed in the skin over the masseter muscle region; an indifferent electrode is placed in the skin over the occiput. After the retina is stimulated with a flash of light, the initial deflection is negative and termed the a-wave. This is followed by a positive deflection (b-wave) which has a greater amplitude and latency. In addition to the a and b-waves, there can be a second positive deflection, the c-wave, which usually is not recordable under standard conditions.
The electroretinogram is a valuable clinical aid in determining retinal function in the presence of an opaque cornea or lens, or normal appearing ocular fundi. The electroretinogram is not a measure of vision. It is a measure of the functional integrity of the entire outer layers of the retina (therefore, the electroretinogram is not useful for small or focal retinal lesions). The standard electroretinogram indicates little about the integrity of the pathways leading from the ganglion cell layer of the retina to, and including, the occipital cortex.
The following are clinical situations in which the electroretinogram is useful:
- Pre-surgical evaluation for cataract extraction and sometimes glaucoma surgery.
- Assessment of retinal function following trauma to the eye.
- Diagnosis of heritable retinal diseases such as progressive retinal degeneration, central progressive retinal degeneration, hemeralopia, and retinal dysplasia (if severe).
Latency - time between stimulus onset and peak of each wave - especially important in the heritable retinal problems (latencies become increased); measured in msec.
Amplitude - 'height' of wave in terms of voltage - amplitudes usually decrease in retinal abnormalities; measured in microvolts.
Dark adaptation (scotopic function) - may be lacking or increased in time to peak in the heritable retinal degenerations.
Flicker response - flashing light is used - responses are 1:1 for certain ranges of flicker frequency - rods and cones differ in these responses:
- Rods - respond to low intensity, low frequency flickering light - generally do not respond 1:1 at frequencies much above 20 hertz (cycles per second).
- Cones - respond to high intensity, high frequency flickering light - generally do not respond 1:1 to low frequency flicker unless intensity is high - upper limit is about 60-70 hertz.
- Flicker response useful in segregating rod from cone function.
Best to do in color - conversion to black and white easy, but not vice versa.
External - full head, side of face, or eye; problem of flash reflection from moist surface (cornea, conjunctiva); try to have flash reflection miss ocular lesion.
Fundus - any portable human fundus camera suitable, but usually economically prohibitive (about $6000 in 1995); also gives good external photos of the eye if the lesion and coloration of the eye and facial skin are light.
This produces regional akinesia of the eyelids by blocking the auriculopalpebral nerve (branch of facial) (Rubin). The primary muscle affected by this block is the orbicularis oculi, which in the horse is powerful and can impede your ability to view the eye due to forced closure of the eyelids. By blocking the action of this muscle, you facilitate examination of the globe. If treatment of a particular horse required manipulation of the eyelids, this block also could be used to facilitate this.
It also can be used in other species (Holmberg, et al.).
Injection is done at caudal aspect of zygomatic arch where there is a palpable notch over which the nerve courses; clean area with alcohol; use 22 gauge (or similar), 2˝ cm long needle; needle is placed subcutaneously (aspirate before injecting - rostral auricular vein close); inject 4-6 ml of 2% lidocaine in a fan-shaped manner - do not massage; successful block indicated by ptosis of ipsilateral eyelid in 10 minutes or less; eyelids should be easy to manipulate even if no ptosis; can repeat in a site little more dorsal and rostral on zygomatic arch; anesthesia is not produced.
Other sites can be used in the horse (Manning and St. Clair).
The following are ophthalmic conditions of horses which should be noted during a pre-purchase (Koch) or insurance exam and which may be considered 'unsoundnesses':
You may want to consult other articles for some general information on material which does not neatly fit into any particular category within this text: animals considered exotic or used in laboratories (Bellhorn; Millichamp, et al.), avian ophthalmology (Buyukmihci; Buyukmihci, et al.; Karpinski and Clubb; Murphy; Stiles, et al.; Tsai, et al.; Willis and Wilkie, a, b), biomicroscopy of the eye (Martin, a, b, c), color vision (Loop, et al.; Neitz, et al.; Riol, et al.), embryology (Aguirre, et al.; Bistner, et al.), heritable ocular disease in general (Wheeler, et al.), 'large' animal ophthalmology (Latimer and Wyman; Leipold; Merideth and Wolf; Vestre; Wyman), melatonin (Czeisler, et al.), ocular emergencies (Bistner and Aguirre), ocular nerve blocks in the cow (Elmore), ocular radiology (Johnston and Feeney), ocular ultrasonography (Dziezyc and Hager), optics (Murphy and Howland; Murphy, et al.), other ideas on examination of the eye (Bistner and Shaw; Moore), and rabbit and rodent ophthalmology (Bauck).
Receptors for the pupillary light response: Although the photoreceptor cells are presumed to be the receptors, many patients with extremely advanced retinal degeneration nevertheless have a good pupillary response to light. It may be that certain classes of photoreceptor cells are involved or that a yet unidentified cell type is responsible for this reflex.
Schirmer II tear test: This test currently is rarely used in veterinary ophthalmology. Prior to applying the test paper, a topical anesthetic is used and the tear film is blotted dry. This test measures basal production as a result.
ERG stimulation method: In this section, I am referring to the standard electroretinogram, one which is evoked by a flash or flashes of light. There also is a pattern-evoked electroretinogram, which could be used to evaluate ganglion cell function, however this is not presently practical to do in a typical veterinary setting.